A challenge in developing high power III-V material-based semiconductor devices, such as light emitting diodes (LEDs), laser diodes (LDs), bipolar junction transistors (BJTs), and heterojunction bipolar transistors (HBTs), is the development of an ohmic contact that has both a low specific resistance and a high current carrying capability. For example, the challenge to manufacture a low resistance ohmic contact to n-type material is particularly important for deep ultraviolet LEDs made from group III-nitride materials, such as Aluminum Gallium Nitride (AlGaN) or Aluminum Gallium Indium Nitride (AlGaInN), which include a high molar fraction of aluminum. Similarly, the challenge for manufacturing quality contacts to p-type nitride semiconductors is important for all nitride-based LEDs since a relatively low p-doping (e.g., less than 1×1018 cm−3) in p-type GaN, which is achievable either by metalorganic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE), makes the formation of such p-type ohmic contacts difficult.
Magnesium (Mg), with a room-temperature activation energy as high as two hundred fifty meV, which increases almost linearly with an increase of the Al molar fraction, is a commonly used acceptor for p-type GaN semiconductors as well as AlGaN and AlGaInN semiconductors with a high molar fraction of Al, such as those used in deep ultraviolet LEDs. For example, a Mg-doped AlGaN/GaN short period superlattice (SPSL), with the period of the superlattice being very small (e.g., below four nanometers), has been proposed to replace a p-type AlGaN semiconductor. In this case, since minibands are formed in the SPSL, vertical conduction of the p-type SPSL should not be degraded compared to that of the AlGaN semiconductor. Several approaches have proposed using Mg-doped AlGaN/GaN SPSL in the growth of 340-350 nanometer ultraviolet LEDs. Mg-doped AlGaN-based large period superlattices (LPSL), with the period of the superlattice being relatively large (e.g., larger than fifteen nanometers), also have been proposed. However, to date, these structures have exhibited a reduced vertical conductivity.
In another approach, a p-type GaN/p-type AlGaN single heterostructure has been used to achieve hole accumulation at an interface. Since such a heterostructure only includes one barrier for hole transportation, the vertical conductivity can be enhanced compared to the LPSL approach due to hole accumulation at the heterointerface, field assisted tunneling, as well as thermal emission. Several approaches for manufacturing deep UV LEDs have incorporated such a heterostructure for hole injection layers.
P-type contact resistivity of 1.1×10−6 ohm-cm2 has been achieved. In particular, a Palladium/Silver/Gold/Titanium/Gold (Pd/Ag/Au/Ti/Au) metallic contact was used under high-current operation for a vertically conducting GaN/InGaN multiple quantum well (MQW) LED structure grown on a Silicon Carbide (SiC) substrate. However, ohmic contacts to p-type nitrides with a high Al composition remain a problem.
To achieve a low n-type contact resistance in a nitride-based device, several contact metals and a relatively high annealing temperature are generally used. To this extent, Al can be used as a contact metal because of its relatively low melting point of approximately 660 degrees Celsius. Furthermore, Titanium (Ti) or Chromium (Cr) can be used as the first layer of the contact due to their low metal work function to nitrides. Specific examples include Ti/Al/Ti/Gold (Au) or Ti/Al/Nickel (Ni)/Au, with thicknesses from five nanometers to five microns and which are annealed at 400 degrees Celsius or higher temperatures. Another approach reverses the order of the Ti and Al, and forms an Al/Ti-based contact to an n-type GaN semiconductor, which includes Al/Ti/Platinum (Pt)/Au and which is annealed at temperatures between 400 and 600 degrees Celsius. Still other approaches form a Cr/Al-based contact to an n-type GaN semiconductor, which include various metal configurations, such as Cr/Al/Cr/Au, Cr/Al/Pt/Au, Cr/Al/Pd/Au, Cr/Al/Ti/Au, Cr/Al/Cobalt (Co)/Au, and Cr/Al/Ni/Au.
Contact reliability also can be a problem. For example, to date, Ti/Al-based n-type contacts for ultraviolet LEDs emitting 265 nanometer and shorter wavelengths have not been shown to be very reliable.